1. Field of the Invention
The present invention relates to a solid-state imaging device in which an organic photoelectric conversion portion is formed on a charge accumulation/transfer/read-out substrate, and provides a solid-state imaging device showing high S/N and a high response speed by specifying a structure of the material.
2. Description of Related Art
Visible light sensors are generally devices prepared by forming a photoelectric conversion site through formation of PN junction within a semiconductor such as Si and, as a solid-state imaging device, a plane type light-receiving device is being widely used in which photoelectric conversion sites are two-dimensionally arranged within a semiconductor, and signals each corresponding to a signal charge having been generated in each pixel by photoelectric conversion are transferred and read out by means of CCD or CMOS. As a technique for realizing a color solid-state imaging device, a structure in which color filters each transmitting only light of a particular wavelength are arranged for color separation on the light incident side of the plane type light-receiving device is general. In particular, as a system at present widely used in, for example digital cameras, a single plate sensor is well known in which color filters each capable of transmitting blue light, green light, or red light are regularly arranged on the two-dimensionally arranged individual pixels.
However, in this system, each of the color filters transmits only light of a particular wavelength, and light not transmitting through the color filter is not utilized, and thus light-utilizing efficiency is bad. Also, with the recent increase in number of pixels, the size of pixel becomes smaller, which leads to reduction in area of the photo diode portion, reduction in aperture ratio, and reduction in light-collecting efficiency.
In order to address the above, it can be considered to stack in the vertical direction photoelectric conversion portions capable of detecting different light wavelengths. As such system with limiting the light to visible light, there have been disclosed, for example, a sensor in which a stacked structure is constituted in the vertical direction utilizing dependency of absorption coefficient of Si upon wavelength to thereby separate colors based on the difference in depth in U.S. Pat. No. 5,965,875, and a sensor of a stacked structure using an organic photoelectric conversion layer in JP-A-2003-332551. However, the absorption ranges utilizing the difference in the depth direction of Si overlap with each other, which leads to insufficient color-separating properties, and thus color separation is poor. Also, as other technique for solving the above, there is known a structure in which a photoelectric conversion layer of amorphous silicon or an organic photoelectric conversion layer is formed on a substrate for reading out signals to thereby increase the aperture ratio.
There have so far been known several examples with respect to photoelectric conversion elements, imaging devices, and photo sensors using an organic photoelectric conversion layer. With them, a high photoelectric conversion efficiency and a low dark current are particularly required and, as techniques for the improvement, introduction of pn junction and introduction of bulk-heterostructure have been disclosed for attaining high photoelectric conversion efficiency, and introduction of a blocking layer has been disclosed for attaining low dark current.
It is true that these structurally improving techniques are effective, but characteristic properties of a material to be used also largely contribute to improvement of element performance. The characteristic properties are one of main factors influencing photoelectric conversion efficiency (exciton dissociation efficiency, charge-transfer properties) and dark current (amount of carrier in the dark) and, though not having been referred to in reports thus-far, are also a factor controlling signal response properties. In the case of using as a solid-state imaging device, it is necessary to satisfy all requirements for high photoelectric conversion efficiency, low dark current, and high response speed. However, no specific proposals have so far been made as to what would be such organic photoelectric conversion materials and element structure.